The instant disclosure relates generally to a surface coating composition for inkjet media.
Digital printing, such as inkjet printing, is rapidly replacing traditional impact printing or “plate” printing methods, such as offset printing. It is sometimes challenging to find media which can be effectively used with such digital printing techniques. To create a superior image with inkjet printing, coated paper is typically used. Such media has single or multiple coating layers with compositions having inorganic or organic pigment as a filler along with other functional materials which promote ink receiving. Papers with coating layers generally show superior physical appearance over uncoated paper in terms of gloss and surface smoothness. In order to achieve higher brightness and whiteness, optical brightening agents (OBAs), also known as fluorescent whitening agents (FWAs), are often added into the coating composition.
To improve the total image quality, metallic salts, such as multi-valent salts like calcium chloride, have been used in surface sizing processing of uncoated plain paper. The salts precipitate out the pigment dispersion from an ink solution so that the pigmented colorant substantially stays on the outermost surface layer of the media. Cations of such salts further fix anionic charged colorants in pigmented ink. This technology increases the optical density and color saturation of the printed images and reduces dry time of such images. It also improves the print quality by sharpening dot edge. One drawback of this technology is the quench effect that these salts have on optical brightening agents (OBAs). OBAs are generally very sensitive to salts, and especially to ionic contamination in salts. The CIE whiteness per the International Organization for Standardization (ISO) method 11475, for example, can drop as much as 3-4 units after adding salts.